The present invention relates to a process for concurrently preparing organosiloxanes and methyl chloride from a corresponding organochlorosilane and methyl alcohol by the reaction expressed by the equation ##STR1##
As is well known, the first and basic step in the production of various kinds of silicones, i.e. polymeric products mainly of organopolysiloxanes, is the hydrolysis of one or a mixture of organochlorosilanes to convert the silicon-bonded chlorine atoms to silanol groups followed by the silanol condensation to give siloxane linkages. the primary product obtained by the above hydrolysis and condensation is usually a mixture of relatively low-molecular organopolysiloxanes (hereinafter referred to as organosiloxanes) and such organosiloxanes are important intermediates for the preparation of high-molecular silicone products by the polycondensation or siloxane rearrangement according to need.
A problem in the above described hydrolysis-condensation reaction of organochlorosilanes is the disposal of the by-product hydrogen chloride formed by the hydrolysis of the silicon-bonded chlorine atoms with water. The major route for the utilization of the by-product hydrogen chloride is for the production of methyl chloride in a separate process in which hydrogen chloride is reacted with methyl alcohol to give water and methyl chloride, which latter is further reacted with metallic silicon to give methylchlorosilanes. Thus, a circuit is formed for the circulation of chlorine through the route of methyl chloride to methylchlorosilanes to by-product hydrogen chloride to methyl chloride. In any way, organosiloxanes and methyl chloride are produced in two separate processes in the conventional silicone technology.
In contrast to the above described conventional processes, there are also known processes for the concurrent or one-step preparation of organosiloxanes and an alkyl halide by the direct reaction of an organohalogenosilane and an alkanol or, in particular, of organosiloxanes and methyl chloride by the direct reaction of an organochlorosilane and methyl alcohol. In theses processes, the chlorine is directly converted to methyl chloride so that the overall costs for the production of silicones can greatly be reduced owing to the absence of the process or facilities for the utilization of the by-product hydrogen chloride such as the separate preparation of methyl chloride by the reaction with methyl alcohol. In addition, advantages are obtained in these one-step processes that the formation of large volumes of hydrochloric acid can be avoided owing to the absence of the step of hydrolysis contributing to the solution of the problem of waste acid disposal and to the saving of energy required for the reaction as well as to the decrease of the loss of materials such as chlorine and methyl alcohol.
Notwithstanding the above described great advantages in principle in the one-step process, the process has not yet been so prevalent in the silicone industry because several problems must be solved before the process is industrially practiced. The principal problems in this process are that the direct alcoholysis reaction between an organochlorosilane and methyl alcohol proceeds at a relatively low velocity so that the productivity of the process is not always satisfactory taking long time, that undesirable side reactions take place to form by-products such as dimethyl ether leading to the decrease in the yields of the organosiloxanes and methyl chloride, that sufficiently high reaction velocity is obtained only by the use of a catalyst and the contamination of the products with the catalyst is sometimes unavoidable and that the facilities for the reaction cannot be so small that the cost for the process is not so low as desired due to the large investment for the facilities.
There have been, of course, made many attempts to solve the above described problems in the one-step manufacturing of organosiloxanes and methyl chloride to establish the practicability of the process. For example, U.S. Pat. No. 2,556,897 describes a process in which a liquid mixture of a methylhalosilane, e.g. dimethyldichlorosilane, and anhydrous methyl alcohol in an excess amount over stoichiometry is kept at an elevated temperature of 40.degree. to 60.degree. C. so that methyl chloride as the reaction product is evolved out of the reaction mixture and collected. This method is simple but the reaction, which is carried out batch-wise, is relatively slow so that the reaction vessel necessarily should be very large when the method is to be practiced in an industrial scale. In addition, the reaction can never be complete, presumably, due to the use of a large volume of methyl alcohol so that the yield of the desired organosiloxanes is not sufficiently high.
U.S. Pat. No. 2,741,630 teaches a process for the vapor phase reaction of methyl alcohol and an alkylhalogenosilane, e.g. dimethyldichlorosilane, on a solid catalyst of zinc chloride supported on a silica gel as the carrier at a temperature of 175.degree. C. or above. Such a high temperature and the use of a strong catalyst such as zinc chloride are not desirable because a side reaction of scission of the bond between the silicon atom and the carbon atom may take place resulting in an unsatisfactory quality of the organosiloxane product even if the yield of the organosiloxanes is not unduly low by suitably selecting the reaction conditions. Indeed, the methylsiloxane products obtained in an example contained only about 50% by weight of cyclic methylpolysiloxanes such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane and the like which are the most valuable constituents in the organosiloxane products.
In contrast to the above mentioned catalytic process, noncatalytic processes are also described, for example, in Japanese Patent Publication No. 50-11368 and U.S. Pat. No. 4,032,557 for the direct reaction of an organohalogenosilane and an alcohol to form organosiloxanes and an alkyl halide. According to the Japanese patent, the organohalogenosilane and the alcohol in liquid forms are introduced in counter-current into a bed formed of inert packings having a specific surface area of at least 0.1 m.sup.2 /g at a temperature of 20.degree. to 150.degree. C. The disadvantages in this process are the low reaction velocity necessitating large-scale facilities when the process is practiced in an industrial scale and the low yield of the alkyl halide due to the loss of the hydrogen halide carried out by the water formed by the reaction.
U.S. Pat. No. 4,032,557 proposes an improvement for the above process by introducing dimethyldichlorosilane and methyl alcohol into a reactor containing at least one layer of packing material which is maintained at a temperature of from 60.degree. C. to 150.degree. C. to form an aqueous phase containing organosiloxanes, and thereafter separating the organosiloxanes from the aqueous phase, in which improvements are obtained by recycling into the reactor from 1 to 6 liters of the aqueous phase for each mole of the dimethyldichlorosilane introduced into the reactor and reacting the organosiloxanes separated from the aqueous phase with dimethyldichlorosilane in an amount from 2 to 10% by weight based on the weight of the organosiloxanes. Substantial improvements are obtained in this method in connection with the reaction velocity over the process described in the above mentioned Japanese patent but the problem of low yield of methyl chloride remains unsolved for the same reasons in addition to the disadvantages due to the further complicated facilities.
Apart from the above described processes for the direct reaction of an organochlorosilane and methyl alcohol in which the reactants either in the liquid or in the vapor phase are reacted without any reaction medium, there are also known processes for the reaction of an organochlorosilane and an alcohol in which the reactants are simultaneously introduced into an aqueous reaction medium to be reacted there.
For example, the process disclosed in Japanese Patent Kokai No. 52-78835 comprises reacting an organochlorosilane and methyl alcohol in an aqueous solution of a Lewis acid such as zinc chloride. It is noted that the reaction conditions described there are far from moderate. In an example, the concentration of zinc chloride in the aqueous solution is as high as 81% by weight and the reaction of dimethyldichlorosilane and methyl alcohol is carried out at 155.degree. C. As a result of such reaction conditions, the amount of the by-product dimethyl ether is remarkably large in the volatile products in addition to the disadvantage of low productivity of the organosiloxanes that only about 2 g of oily organosiloxanes are obtained in each hour by use of a bubble column of 6.5 cm diameter and 110 cm length.
Further, Japanese Patent Kokai No. 54-63028 discloses a process for the direct reaction of an organochlorosilane and methyl alcohol in a reaction medium. In this case, the reaction medium is a melt of a quaternary ammonium salt such as N-methylpyridinium chloride. This process is also disadvantageous due to the low conversion of the chlorine content in the organochlorosilane to methyl chloride. In an example for the reaction of dimethyldichlorosilane with methyl alcohol, the conversion is only 76% by use of a single sparger column and a satisfactory conversion of about 99% is obtained only by the use of 4 sparger columns connected in series.